A role for generalists in the pollination of Krameria erecta oil flowers in central Arizona?

Background/Question/Methods

The desert shrub Krameria erecta (Krameriaceae) belongs to a family of plants associated with a specialized lineage of pollinators, floral oil-collecting bees in the tribe Centridini (Apidae: Apinae).  While Krameria species are generally thought to be dependent on centrid bees for successful pollination, there have been no in-depth studies of which insect species visit and effectively pollinate K. erecta.  Preliminary observations in central Arizona suggested that, in addition to oil-collecting bees in the genus Centris, small sweatbees (Halictidae) also visit K. erecta flowers.  We conducted two experiments at a study site near Sedona in central Arizona to estimate the contribution of different insect visitors to pollination and fruit set in this K. erecta population.  In the pollinator exclusion experiment, nylon bags in various mesh sizes were used to manipulate which insect types could reach flowers.  One branch on each experimental plant (n=25) was assigned to each of three treatments: Closed  (<1 x <1mm mesh bag that excluded all potential visitors),  Centris-excluded  (4 x 4mm mesh bag through which halictid bees moved freely), and Open (no bag).  In the single visit experiment, two types of unvisited flowers were exposed to pollinators:  female-only flowers from which anthers had been removed, and intact flowers with mature anthers present.  Once a flower was visited by an insect, the stigma was examined for pollen deposition.

Results/Conclusions

As expected, K. erecta flowers at our study site were visited by female Centris collecting floral oil.  Flowers were also frequently visited by female Halictid species, Lasioglossum subgenus Dialictus and Halictus tripartitus, collecting pollen. Results of the pollinator exclusion experiment suggest that, despite frequent floral visits by sweatbee species, Centris bees account for the majority of fruit set in our K. erecta study population.  Centris-excluded branches had a mean fruit set of 5 (± 1)%, while branches open to all potential pollinators had a mean fruit set of 36 (±4)%.  Closed branches had a low level of fruit set (3 (± 2)%), but none of the fruit showed normal ovule development.  Results of single visits to female-only flowers indicated that both Dialictus and Centris move K. erecta pollen among flowers and deposit this pollen on stigmas, though Dialictus tended to deposit fewer grains per visit.  Our preliminary results suggest that K. erecta pollination ecology may be more complex than expected, with the potential for some low level of pollination by insects other than specialized oil-collecting bees.